The present invention relates to a pulsed ultrasound Doppler blood flow sensing device of the type in which ultrasound pulses are emitted from a transducer at a fixed rate frequency into an organism, echoes from blood corpuscles are received by the transducer, Doppler frequency shifts of the ultrasound pulses are detected from the echoes, and a blood flow is displayed by a display device.
The pulsed ultrasound Doppler blood flow sensing device measures a blood flow by making use of a proportional relation of the blood flow to the Doppler frequency shift.
Generally, the Doppler frequency shift f.sub.d is given by the following equation ##EQU1## Where f.sub.c : Frequency of a ultrasonic pulse radiated toward a moving object,
V: Velocity of the moving object which refelects the ultrasound pulses, PA1 .theta.: Angle of the ultrasonic pulse to the moving direction of the moving object, PA1 C: Propagation velocity of the ultrasound pulse.
From the above equation, it is seen that the Doppler frequency shift f.sub.d is proportional to the velocity of the moving object. The pulsed ultrasound Doppler blood flow sensing device depends on the above relation of the Doppler frequency shift to the velocity of the moving object. In the blood flow sensing device, ultrasound pulses are radiated from a transducer at a fixed rate frequency or a fixed sampling frequency. Echoes of the radiated ultrasound pulses from blood corpuscles (the moving object) are received by the transducer to obtain the Doppler frequency shifts of the ultrasound pulse on the basis of the frequencies of the echoes. The blood flow is measured on the basis of the Doppler frequency shifts.
The sampling theorem describes that the upper limit of the Doppler frequency shift is 1/2 of the sampling rate frequency f.sub.r. For this reason, the prior device converts the received echoes into electrical signals (echo signals), passes the component of the echo signals coming from the corpuscles in the blood flowing in a toward (+) direction through a band pass filter with a frequency band from 0 to f.sub.r /2 and analyzes the frequency of the signal passed through the filter. The component of the echo signals of the corpuscles in the blood flowing in an away (-) direction is passed through a band pass filter of 0 to f.sub.r /2 and the frequency of the signal passed through the filter is analysed. Only the signal as shown in FIG. 1 within a frequency range between +f.sub.r /2 and -f.sub.r /2 is displayed as the blood flow signal Incidentally, the term "toward" direction means the direction of the blood flow flowing toward the ultrasound source or the transducer, and the "away" direction is opposite to the "toward" direction.
The band pass filters assembled into the prior blood flow sensing device have each a frequency characteristic in which the attenuation degree is extremely large in the vicinity of the frequency of .vertline.f.sub.r .vertline./2, as shown in FIG. 2. Accordingly, the prior device displays only the blood flow signals that fall within a frequency band defined by .+-.f.sub.r /2, and cannot display the blood flow signal at a frequency exceeding .+-.f.sub.r /2. Therefore, it is difficult to correctly recognize the blood flow signal.